I would say Heavy Rain because its short (still kind of frustrating at parts) then pick a longer title like the witcher and when you need to take a break from that play the walking dead. I also have a massive back log my problem is most of them are fairly long Jrpgs

Finish off the Walking Dead, the entire game can be finished quickly and will be a great time. After that I would go with AC3. Generally when I have a lot on my plate I play the games that are very different. For instance I just finished playing Blue Dragon and now am digging into Dead Space 2. This helps me to keep from getting genre fatique which in the past has caused my backlog to grow exponentially.

I would start with The Walking Dead also problem because it's so good & shorter than the rest.

A good tip at least for me, is that when I start one game I make sure I finish the single-player portion before moving on to another game. It actually increases the chance that you'll complete whatever you're playing & definitely helps you focus on one narrative instead of juggling multiple stories in the air. Hope that helps!